1. Field of the Invention
The present application relates generally to coatings. More specifically, the present application relates to multilayer coatings including an alumina layer.
2. Background of the Invention
In the discussion of the state of the art that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention.
From U.S. Pat. No. 6,221,469, it appears that the use of a xcexa-Al2O3 coated metal cutting insert is better than an xcex1-Al2O3 coated cutting insert, particularly where high-speed turning is concerned. However, it has been found that in interrupted turning and in turning with coolant, the xcex1-Al2O3 coated insert performs better. This appears to be so because between the xcex1- and xcexa-forms, the a form is more ductile.
xcexa-Al2O3 cannot deform plastically due to its defect structure. When the application temperature is lower, e.g., with coolant, xcexa-Al2O3 cannot transform to xcex1-Al2O3 either. On the other hand, the temperature and especially the pressure are obviously high enough to activate sufficient amount of slip systems in the xcex1-Al2O3 phase. Consequently, the insert containing the xcexa-Al2O3 is more brittle due lack of plasticity during cutting.
It is an object of this invention to avoid or alleviate the problems of the prior art.
It is further an object of this invention to provide a particularly efficacious coated cemented carbide body for the cutting of steel or cast iron.
An exemplary embodiment of a coated sintered cemented carbide body comprises a cemented carbide body, a first layer adjacent the cemented carbide body, the first layer including Ti(C,N) and having a thickness of from about 3 to about 20 xcexcm, an alumina layer adjacent said first layer, the alumina layer including xcex1-Al2O3 or K-Al2O3 and having a thickness of from about 1 to about 15 xcexcm, and a further layer adjacent the alumina layer of a carbide, carbonitride or carboxynitride of one or more of Ti, Zr and Hf, the further layer having a thickness of from about 1 to 15 xcexcm.
In an exemplary embodiment of a coated sintered cemented carbide body, a friction-reducing layer is adjacent to the further layer, the friction-reducing layer including one or more of xcex3-Al2O3, xcexa-Al2O3 and nanocrystalline Ti(C,N). The friction-reducing layer has a thickness of from about 1 to about 5 xcexcm.
In one aspect, the coated sintered cemented carbide body, in which the Al2O3 is xcex1-Al2O3, can be used to cut cast iron.
In another aspect, the coated sintered cemented carbide body, in which the Al2O3 layer is xcex1-Al2O3, can be used for the cutting of steel.